Neonatal Immunoglobulin Secretion and Lymphocyte Phenotype in Rat Small Intestine Lamina Propria

We characterized the lymphocyte phenotype and the ability to produce Ig by lamina propria (LP) cells from rat ileum throughout the suckling period. In the first week after birth,10% of LP lymphocytes were B cells, but at weaning, this figure rose to30% as found in the adult. These B cells did not bear surface IgA (sIgA-). However, the number of sIgA+, which may correspond to B blast cells because they were outside lymphocyte cytometer gate, increased. In LP, IgM-secreting cells (SC) appeared during the second week of life, and IgA-SC were detected later but at a lower number. Regarding LP T cells, CD8+ cells were more abundant than CD4+ cells along the first 2 postnatal weeks, and CD3+CD8alphaalpha+TCRalphabeta+CD5-CD25- was their predominating phenotype. In this 2-wk period, between 8 and 20% of LP were natural killer cells. LP CD4+ lymphocytes in neonatal rats showed increasing co-expression of TCRalphabeta, whereas the co-expression of CD90 decreased and the CD4+CD25+ cell percentage did not achieve adult values. In conclusion, in the first 2 wk of the rat life, the gut LP immune system shows abundant CD8alphaalpha+ cells, including NK cells. Thereafter, LP B cells increase dramatically and Ig-SC appear, with IgM-SC being more abundant than IgA-SC. CD4+ LP lymphocytes acquire a mature phenotype and adult proportions later after weaning.